Bava Kama 6a - nezikin learned out from 2 avos

There is a machlokes rishonim what is the status of Nezikin that are learned out from multiple avos.
This is discussed in the Rosh and expounded in the Griz al Harambam in the beginning of Hilchos Nizkei Mammon.
The question is as follows: אבן,סכין ומשא  that fell down and cause damage after landing, we learn according to Shmuel that if the owner was not mafkir, then it is learned out from a combination of esh and bor.
Tosfos understands this to mean that it will have the halachos of what it was learned from, i.e. this will have on the one hand a petur of tamun like esh and on the other hand, like bor that adam and and keilim are patur.
However, there is an alternative understanding to this The Rosh explains that אבן,סכין ומשא, once the learning is complete from esh and bor, are then completely like bor. Esh was used to learn out that one is chayav but it is a pure bor in terms of halacha. This actually also works well with what we learned in shiur that even according to Rav, he also really learns out even sakin umasa from a combination of bor and shor (mentioned as shitas Rashbam in tosfos 3b), just acc to him, after we learn it, they are pure shor as opposed to bor in terms of the halacha. And according to Shmuel, it is a pure bor after the derivation.
The Griz explains this approach as follows: the petur of tamun is a גזירת הכתוב and limited to only an item that has a שם אש but something that אש was used to learn it out does not make it אש and therefore the petur does not apply. This would be true in a reverse case as well, where the item is defined as esh and bor was used to learn it out, i.e אבן,סכין ומשא that cause damage while airborn. However, Tosfos disagrees and says whatever is the source to learn out the din that you are chayav, must also reflect on the parameters of what you are chayav for.
The Rosh's explanation also enables us to have an alternate peshat to Tosfos 3b on top that says we can only learn from keren to even sakin umasa since only keren is chayav in רשות הרבים. But if we say that the נלמד does not have to have all the halachos of what it learned from applied to it, then we are ok to learn even from shen or regel.
Similarly, the Griz explains that תולדותיהם כיוצא בהם או לאו כיצא בהם according to the Rosh is really asking the following: do the toldos of the avos have the exact status as the avos in terms of all the halachos that are unique to each one or not, or are they merely learned out from the av but they dont have the same שם as the av and therefore would not have all the unique dinim. And the answer of the gemara is that since all the toldos have all the characterstics of the av, therefore they have all the unique halachos as well.